How the Brain Works The Facts Visually Explained by DK (z-lib.org)

56

THE PHYSICAL BRAIN

Genetics and the Brain 57

How faulty genes affect the brain

Genes do not directly control behavior; instead, they

govern the number and characteristics of nerve cells

whose actions combine to produce our mental

functions. For example, some genes influence the

levels of neurotransmitters (see p.24), which in turn

regulate functions such as memory, mood, behavior,

and cognitive skills. A faulty gene may fail to produce

a protein needed for healthy brain function or may

increase the risk of a disorder such as Alzheimer’s

disease. Some faults can be inherited from parents;

two inheritance patterns are shown here.

AT LE A S T ONE-THIRD

OF ALL OUR GENES

ARE ACTIVE PRIMARILY

IN THE BRAIN

When cells divide, the double-stranded DNA splits into

single strands, and each base is matched with a new

complementary base to form two new copies of the

DNA. However, sometimes copying produces changes

in the sequence. These may cause a gene to produce an

altered protein or stop it from working at all. Mutations

may arise during life or may be inherited from parents.

MUTATION

Base

pair

Backbone

of DNA

molecule

Mutation occurs when

base pairs are changed

during copying

New DNA strand made

during cell copying ERROR

Autosomal dominant

In an autosomal dominant

disorder, such as Huntington’s

disease, only one parent has

to pass on the faulty gene for

it to cause the disease.

Autosomal recessive

In an autosomal recessive

disorder, such as Tay-Sachs

disease, the disorder occurs

only if both parents pass on

a faulty copy of the gene.

Carriers have no disease

themselves but can pass on

the faulty gene.

Guanine (blue) always bonds

with cytosine (green)

AFFECTED

PARENT

CARRIER

PARENT

UNAFFECTED

PARENT

CARRIER

PARENT

Faulty gene

present

Affected child

has two copies

of faulty gene

Carrier children

have one faulty and

one healthy gene

Unaffected

child

Normal

gene only

Parent has

one faulty

and one

healthy gene

AFFECTED

CHILDREN

UNAFFECTED

CHILDREN

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